Solid freeform fabrication is a process for manufacturing three-dimensional objects, for example, prototype parts, models and working tools. Solid freeform fabrication is an additive process in which an object, which is described by electronic data, is automatically built, usually layer-by-layer, from base materials.
Several principal forms of solid freeform fabrication involve a liquid ejection process. There are two main types of solid freeform fabrication that use liquid-ejection: binder-jetting systems and bulk-jetting systems.
Binder-jetting systems create objects by ejecting a binder onto a flat bed of powdered build material. Each powder layer may be dispensed or spread as a dry powder or a slurry. Wherever the binder is selectively ejected into the powder layer, the powder is bound into a cross section or layer of the object being formed.
Bulk-jetting systems generate objects by ejecting a solidifiable build material and a solidifiable support material onto a platform. The support material, which is temporary in nature, is dispensed to enable overhangs in the object and can be of the same or different material from the object.
In both cases, fabrication is typically performed layer-by-layer, with each layer representing another cross section of the final desired object. Adjacent layers are adhered to one another in a predetermined pattern to build up the desired object.
In addition to selectively forming each layer of the desired object, solid freeform fabrication systems can provide a color or color pattern on each layer of the object. In binder-jetting systems, the binder may be colored such that the functions of binding and coloring are integrated. In bulk-jetting systems, the build material may be colored.
Inkjet technology can be employed in which a number of differently colored inks are selectively ejected from the nozzles of a liquid ejection apparatus and blended on the build material to provide a full spectrum of colors. On each individual layer, conventional two-dimensional multi-pass color techniques and half-toning algorithms can be used to hide defects and achieve a broad range of desired color hues.
One of the on-going deficiencies of the solid freeform fabrication techniques described above is that by building the object with discrete layers, the layers may still be apparent in the finished product. This is especially an issue with objects that have vertically contoured surfaces where the contours spread across multiple layers and create a layering artifact. This is commonly described as “terracing.” The terracing effect leaves noticeable visual and textural “stair steps” at each successive layer along a contour.
This phenomenon is illustrated in FIGS. 1a-c. As shown in FIG. 1a, it may be desired to form a smooth, contoured outer surface (100) of an object. However, the contoured surfaces must be built of stacked layers of build material. Therefore, typical freeform fabrication techniques create discrete layers (102) that attempt to approximate or match the desired surface (100) contour. As shown in FIG. 1b, the build layers (102) are arranged like stairs in an attempt to approximate the desired surface contour (100).
Consequently, the resolution of prior freeform fabrication techniques is limited by the thickness (T) of the layers (102). The actual shape (104, FIG. 1c) of the surface may be a noticeably terraced set of distinct layers (102) instead of a smooth contour as desired.
One solution to the terracing problem is to use thinner layers to build the object. As the layers become thinner, the terraces become shallower and thus less distinct and noticeable. However, by adding additional layers, the throughput of the system is reduced and the object production speed is significantly diminished. The more layers that are needed to build a product, the more time it takes to build that product.
Furthermore, by using thinner layers to produce an object, the data that must be sent to the fabrication system increases. If, for example, the layer thickness is reduced by half, the number of layers (and the data defining those layers) doubles.
In some instances, the data cannot be sent to a fabricator at a high enough rate to enable efficient production of the thinner layers. Thus, even if the decision is made to use thinner layers to reduce terracing, the data sometimes cannot get to a fluid ejector at a high enough rate to result in efficient object production. In these cases, either the fabricator is slowed down to allow the data to transfer, or the same data may be erroneously used to build more than one layer.